18.1 Reading Plumbing Isometrics and Riser Diagrams

Five items, drawn instead of written

Isometric Analysis is worth 5 scored items on the Colorado Journeyworker exam. The items are not drafting questions. They are code questions — trap arm length, vent connection height, fitting selection, stack sizing, cleanout location — asked by showing you a drawing and asking what is wrong with it, or what size a marked segment must be.

That means the skill to build is translating a picture back into the code sections that govern it.

How an isometric is constructed

A plumbing isometric is a pictorial drawing, not a perspective drawing:

• The two horizontal axes are drawn at 30 degrees above the horizontal, one going up-left and one going up-right.
• The vertical axis is drawn true vertical.
• All three axes are drawn to the same scale. Nothing shrinks with distance.

The consequence is that a single line on the page can be read as a real length. A 6-foot trap arm drawn on the up-right axis is 6 feet, measured along the line.

What the three directions mean

Any line not on one of those three axes is an offset — a pipe running diagonally through the building. Offsets are where the trigonometry of Section 18.3 comes in.

What a plumbing isometric shows

A complete DWV isometric identifies, for every run:

1. Fixtures, by symbol or label — WC (water closet), LAV (lavatory), SH (shower), TUB, KS (kitchen sink), FD (floor drain), LT (laundry tray).
2. Traps, drawn as a U or labeled P-trap.
3. Fixture drains and trap arms, from each trap to the branch.
4. Horizontal branch drains.
5. Stacks — soil stacks, waste stacks, vent stacks.
6. Vents — individual, common, branch, wet, circuit.
7. The stack vent and its roof terminal.
8. The building drain and the point where it becomes the building sewer.
9. Cleanouts.
10. Pipe sizes, marked on each segment.
11. Direction of flow, usually by arrowheads.

IPC 316.1.4 lists essentially the same content for an alternative engineered design submittal: floor plans and a riser diagram showing "the direction of flow, all pipe sizes, grade of horizontal piping, loading and location of fixtures and appliances."

Isometric versus riser diagram

For questions about trap arm length, developed length or offsets, you need the isometric. For questions about branch intervals, vent stack connections or which fixtures are above an offset, the riser diagram is the faster read.

Common symbols and line conventions

The solid-versus-dashed convention is the one to fix in mind: on most plumbing isometrics, if the line is dashed, it is a dry vent and everything in IPC Chapter 9 applies to it — connection above the centerline (905.3), rise to 6 inches above the flood level rim (905.4), grade back to the drain (905.2), and size at half the drain served (906.2).

A reading routine

Work an isometric in this order and nothing gets missed.

1. Find the building drain and the direction of flow. Everything else is upstream of it.
2. Find the stacks. Trace each one from the building drain to its terminal. Is the top of it a stack vent (the same pipe continuing above the highest branch) or is there a separate vent stack alongside?
3. Count branch intervals on each stack. Five or more means IPC 904.2 requires a vent stack; more than ten means IPC 908 requires relief vents at each tenth interval.
4. Work fixture by fixture. For each: is it trapped? Is the trap the right size (Table 709.1)? Is the trap arm within Table 909.1? Is the vent takeoff more than two pipe diameters from the weir (909.3) and not below the weir (909.2)?
5. Name the venting method for every trap. Individual (910), common (911), horizontal or vertical wet (912), waste stack (913), circuit (914), combination waste and vent (915), island (916), single stack (917), or an air admittance valve (918). If you cannot name it, it is a violation of 901.2.1.
6. Check every fitting against Table 706.3 for the change of direction it makes.
7. Accumulate dfu along each branch and stack, and check against Table 710.1(1) or 710.1(2) — reading the correct table for the piece of pipe.
8. Check cleanouts: every 100 feet, at horizontal direction changes greater than 45 degrees, and at the building drain/sewer junction (708).
9. Check the terminal: the method (903.1), the location relative to openings (903.5), and, under the model code, frost closure (903.2).

Worked read: a two-bathroom isometric

The drawing shows, from the building drain upward:

• A 4-inch building drain running left to right, with a cleanout at the left end.
• A 3-inch soil stack landing on the building drain through a long sweep, 38 inches downstream of the cleanout.
• At the first floor, a 3-inch horizontal branch picking up a WC, a LAV and a TUB.
• At the second floor, an identical branch.
• Above the second-floor branch, the stack continues as a 3-inch stack vent through the roof.
• A dashed 1-1/2 inch line from the first-floor lavatory trap arm rising and tying into the stack vent.

What to check:

That table is what an Isometric Analysis item looks like from the inside: one drawing, eight code sections.

Colorado field note (not exam-scored): Board Rule 1.6(B)(2)(c) requires an approved set of design plans for commercial and school jobs to be located at the job site. Not having the isometric on site is a reinspection-fee trigger under Rule 1.6(B)(5).